When fueling large airplanes at the loading/unloading aprons of major airports, fuel is usually supplied at substantial pressure by underground mains to fuel hydrants located at each apron. A fuel cart is rolled up to the airplane and is hooked between the nearest fuel hydrant and the fuel filler connection on the airplane.
The cart performs several services ancillary to providing to transferring fuel, for example, a fuel cart has hose reels, filters, water separators, and a flow meter to measure the quantity of fuel boarded, for billing. Many of these and other ancillary services require the availability of a quantity of compressed air. This is customarily provided by compressed-air storage bottles carried on the cart which are a nuisance to check often and recharge. It would be more convenient if the air could be compressed right on the cart without the weight, noise, and inconvenience of an on-board engine or the added complexity and safety risk of an electrical connection near the fume-laden apron.
There has been some use of the pressurized flow of the fuel from the hydrant to the airplane in order to run some ancillary fueling services. One example is using a fluid motor driven by the flow of pressurized fuel obtained from the apron hydrant and delivered to the airplane in order to drive a fuel-additive injector.
However, in fueling an airplane, the fuel flow is not constant. It slows greatly as the tank nears full. The resulting variable fluid motor output is quite acceptable for a fuel-additive injector which should operate at a rate commensurate with the rate of fuel flow. However, something like an air compressor is best run at a constant speed during the entire fueling operation.